This invention relates to novel room temperature coalescable aqueous fluoropolymer dispersions and a process for their manufacture. These dispersions may be used in a variety of applications including high temperature thermoplastic coatings, polishes, room temperature formable paints and coatings, extrudable or injection moldable resins, and adhesives.
Fluoropolymers are well known for their exceptional barrier properties, high temperature stability, chemical resistance, and weather resistance. However, the use of fluoropolymers based on tetrafluoroethylene ("TFE"), chlorotrifluoroethylene ("CTFE"), vinylidene fluoride ("VDF"), fluorinated ethylene-propylene ("FEP"), perfluorinated vinyl ethers, or perfluorinated acrylates in coating applications is limited as a result of the high temperatures required to fuse dispersions of these polymers onto various substrates.
Two well-known methods for preparing low temperature coalescable coatings incorporate either (1) non-crystalline or amorphous fluoropolymers which are dispersible in water; or (2) fluoropolymers which are soluble in organic solvents. These methods are not without their shortcomings. As disclosed for example in U.S. Pat. No. 4,631,326, the first type of fluoropolymer has low fluorine content, possesses poor barrier properties and undesirable weatherability characteristics. The costs incurred with handling large quantities of volatile organic chemicals ("VOC") make the second type of fluoropolymer unattractive. Moreover, when these polymers possess less than 40% by weight of fluoroolefin, their barrier, weatherability, and stain resistance properties are decreased, while their susceptibility to yellowing is increased. By contrast, when these polymers possess greater than 80% by weight of fluoroolefin, their solubility in common organic solvents, as well as their ability to polymerize with non-fluorinated olefins is reduced which leads to a low yield of low molecular weight product. Solvent-based fluoropolymers having a high solid content also suffer from an increased viscosity, which makes their processing and subsequent use as coatings more difficult. See U.S. Pat. No. 5,304,617. In addition, not only do many of these solvent-born fluoropolymers contain expensive comonomers such as cyclohexyl vinyl ether, but most of them swell in various solvents.
Methods for polymerizing core/shell fluoropolymer compositions are well-known in the art. See e.g., U.S. Pat. Nos. 4,952,636, 4,952,630, 4,904,726, 4,036,802, 4,780,490, 5,188,764 and 5,296,165. U.S. Pat. No. 4,469,846 describes a core/shell polymer containing a crystalline, non-elastomeric core of either TFE, VDF, or CTFE and an elastomeric copolymer shell of either VDF or TFE with HFP, TFE, perfluorinated alkyl ethers, or propylene. While these highly crystalline compositions (&gt;50%) modify the physical properties and ease of processing of TFE-based resins, they are not designed to be coalescable at room temperature, nor is there a disclosure provided therein regarding methods for curing or crosslinking these compositions at room temperature. Room temperature coalescability is an important property for fluoropolymer dispersions used in applications such as coatings and/or paint applications since such dispersions have an economically-advantageous, reduced cycle time for curing which does not require the use of heat.
U.S. Pat. Nos. 4,851,479 and 4,946,900 disclose the polymerization of a shell of polyvinylidene fluoride ("PVDF") or copolymers thereof onto a CTFE-VDF copolymeric core in order to improve the mechanical flexibility properties over those of pure PVDF. However, although the resulting composition yields a high melting resin (160-170.degree. C.) which, after processing possesses improved flexibility over that of pure PVDF, the composition disadvantageously does not possess low temperature coalescability.
The preparation of room temperature coalescable fluoropolymers, is not without its difficulties. It is desirable to incorporate into the fluoropolymer backbone a large quantity of a hydrophilic cure site provider such as 2-hydroxyethylacrylate in order to enhance mechanical properties after film formation. However, the copolymerization of hydrophilic olefins with hydrophobic fluoroolefins often leads to latex instability, causing coagulation. As described in Japanese Patent 228491 and European Patent 0675182A1, this problem can be overcome by conducting the polymerization in the presence of large quantities of nonionic and ionic surfactant. However, moisture sensitivity and chemical resistance are compromised by this process.
Therefore, there continues to be a need for water-based fluoropolymer dispersions capable of coalescing into continuous films at or below room temperature in the absence of large quantities of nonionic or anionic surfactants. A continuing need also exists for dispersions which: 1) possess a glass transition temperature low enough to allow for film formation when applied to a substrate while remaining stiff upon curing; 2) are solvent resistant after cure and non-tacky so that contaminants do not adhere to the resulting film; 3) are partially crystalline to impart adequate barrier properties, but flexible enough to undergo some mechanical deformation; and 4) have high enough fluorine content to provide acceptable weatherability.